We propose to develop novel photoacoustic endoscopy by miniaturizing photoacoustic imaging probes. The primary motivation is to overcome the depth limitation of existing endoscopic imaging technologies and to provide functional information sensitive to disease states. The improved imaging capabilities have the potential for early detection of cancer in the gastrointestinal tract. In a preliminary study, we demonstrated the feasibility of photoacoustic endoscopy through in situ and ex vivo animal experiments with our endoscopic probe prototype. We will show its full endoscopic imaging potential and develop broader application through various in vivo animal and human experiments. Additionally, we will advance the current technology by constructing smaller endoscopic probes that fit into generic endoscopes and by improving overall system performance. The specific aims of this project are as follows. Aim 1. Develop a next-generation photoacoustic endoscope system. We will develop a next-generation photoacoustic endoscopic system and improve the image resolution, field of view, scanning speed, and probe size. We will establish the necessary supporting, peripheral subsystems including a laser source and light delivery path, a stepper motor drive, a data acquisition subsystem, and a master control of all subsystems. Aim 2. Design and develop a piezoelectric ring probe of improved sensitivity. We will design and engineer ultrasonic transducers optimized for the proposed photoacoustic endoscope. The ultrasonic transducer is an essential component of the photoacoustic endoscopic system. The optimization of photoacoustic endoscopy depends on several transducer parameters: size, noise figure, and sensitivity. Aim 3. Validate the endoscopic system through phantom and animal experiments. Through phantom experiments, we will validate the performance of the endoscopic system by measuring the spatial resolution, imaging depth, signal-to-noise ratio, and frame rate. Moreover, we will demonstrate its endoscopic imaging potential through various animal experiments. Parts of the gastrointestinal tract, including the esophagus, large intestine, and rectum, and/or parts of the cardiovascular system of animals, will be imaged in vivo or ex vivo. Aim 4. Image Barrett's esophagus in vivo. First, we will image a series of human esophagus in patients with an established diagnosis of Barrett's esophagus to fine tune the photoacoustic imaging system while simultaneously obtaining mucosal biopsies of the distal esophagus. Second, we will compare the targeted photoacoustic images to the ex vivo histology of esophageal mucosal specimens to develop a classification system for photoacoustic images of Barrett's epithelium. Lastly, we will prospectively assess the agreement between the photoacoustic imaging system and standard clinical practice of 4 quadrant esophageal biopsy in a comparative study. The hypothesis is that ultrasound and photoacoustic imaging technologies in combination provide sufficient spatial resolution and contrast to diagnose Barrett's epithelium and Barrett's-associated neoplasia with high sensitivity and specificity.